1. Field of the Invention
The present invention relates to a printing apparatus and, more particularly, to a printing apparatus than prints the two surfaces of a printing sheet using dedicated inkjet printheads.
2. Description or the Related Art
In general, a printing apparatus functioning as a printer or the printing unit of a copying machine or facsimile apparatus forms an image (including a character, symbol, and the like) on a sheet-like printing medium such as a paper, cloth, or plastic sheet using a printhead based on image information. The printing methods of printing apparatuses can be classified into a serial printing method and a full-line printing method by the type of the printhead used, in serial printing, an image is printed by alternately repeating an operation or printing while moving a serial printhead along a printing medium in a predetermined, direction (main scanning direction) and an operation of conveying the printing medium at a predetermined pitch in a direction (sub-scanning direction) perpendicular to the main scanning direction. On the other hand, in full-line printing, an image is printed by conveying a printing medium while printing one line at once using a full-line printhead. Printing apparatuses can be classified into, for example, an inkjet printing apparatus, a thermal transfer printing apparatus, an electrophotographic printing apparatus, a thermo-sensitive printing apparatus, and a wire dot printing apparatus by the printing method.
Especially, inkjet printing apparatuses using a full-line printhead feature a high printing speed and are widespread as output devices mainly used for business purposes. There has also been proposed an arrangement including printheads dedicated to print the front and back surfaces to speed up two-sided printing. An arrangement that provides one subtank in correspondence with each ink type regardless of the number of printheads used for downsizing and cost reduction is described in, for example, paragraph [0083] and FIG. 8 of Japanese Patent Laid-Open No. 2010-64389.
In the conventional inkjet printing apparatus, however, the water head difference between the printhead connected to one subtank and used to print the front surface of a printing sheet and the printhead used to print the back surface generates a negative pressure difference in the heads, leading to the difference in the ink discharge amount. To solve this problem, the ink supply channel of the printhead having a smaller water head difference is made to have a smaller sectional area than that, of the other ink supply channel, thereby generating a difference in the resistance to flow of ink and stabilising the negative pressures in both printheads.
However, in this arrangement, the resistance to flow of ink is proportional to the square of the flow velocity of ink. For this reason, the same negative pressure can be generated in both printheads only at a specific ink flow rate. When the print data amount changes, the ink amount to be consumed changes, and the Ink flow rate chances. Hence, a density difference between images printed on the two surfaces of the printing sheet might occur, depending on the image.